Dan: How much time and money would you say FusionAuth has saved you?
Jason: A significant amount. Compared to rolling out our own authentication and authorization services, we estimate that it saved us around 1000 developer hours. Using a standard $100/hour rate, that’s $100k in savings.
As for competing services, we believe FusionAuth saved us around $300-$800 per month for the 12+ months we’ve been using it.
I will try to offer a few pointers, but as always, the juice is in the details. If you find yourself wanting more guidance, we do offer support plans to assist with implementation (not an upsell, just an option if needed per business requirements )
You could add Your Custom SSO as OpenID Connect Identity Provider. My thinking is that you could have FusionAuth issue an access token by visiting a login page and have your custom SSO accept that token, verify it, and then offer access to all the applications behind its "gate". In the picture from the documentation, replace pied piper with custom sso login button. Disclaimer: I may have to think more on this one to see if it best applies.
I will see if @dan, my colleague, has any other directional pointers to add or if I anything else pops to mind.
Many people use connectors when doing a staged migration of a system (documentation here) as you are. That would be a workflow to consider and educate yourself on. Specifically, there is a section in the documentation on Authentication that might be helpful to you.